Optical transmitters that can pre-compensate for impairments of an optical link are described in Applicant's co-pending U.S. patent application Ser. Nos. 10/262,944, filed Oct. 3, 2002; 10/307,466 filed Dec. 2, 2002; 10/405,236 filed Apr. 3, 2003; and 10/677,223, filed Oct. 3, 2003, the contents of all of which are hereby incorporated herein by reference, and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003. Techniques for compensating both linear and non-linear impairments in an optical link by using an optical transmitter capable of pre-compensating for optical link impairments are disclosed.
An example of such an optical transmitter is shown in FIG. 1. In the transmitter 100, a signal processor 16 receives the input data signal x(t) as an input, and uses a compensation function to compute successive multi-bit In-phase and Quadrature values representing successive loci of the end-point of a desired or target optical E-field vector. A linearizer 18 then uses the multi-bit loci to synthesize a pair of multi-bit digital drive signals. The digital drive signals are then converted into analog (RF) signals by respective high speed multi-bit Digital-to-Analog Converters (DACs) 20, which are then amplified 21 (and possibly band-pass filtered to remove out-of-band noise) to generate the drive signals supplied to an electrical/optical (E/O) converter 22. The E/O converter 22 will normally be provided as either nested Mach-Zehnder (MZ) interferometers, or as a conventional dual branch MZ interferometer. The E/O converter 22 modulates a carrier signal 6 having a desired wavelength is generated by the laser 2. The digital drive signals are computed such that the drive signals supplied to the E/O converter 22 will yield an optical E-field EO(t)8 at the E/O converter output 24 that is a high-fidelity reproduction of the target E-field computed by the signal processor 16.
In general, the signal processor 16 is capable of implementing any desired mathematical function, which means that the compensation function can be selected to compensate any desired signal impairments of the optical path, including, but not limited to, dispersion, Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), four-wave mixing and polarization dependent effects (PDEs) such as polarization dependent loss. In addition, the compensation function can be dynamically adjusted for changes in the optical properties of the link and component drift due to aging.
In the commissioning or initialization of optical communication links between nodes of an optical system, the impairments of the optical communications link, such as dispersion, must be determined so that the transmitter of the link can be appropriately configured to overcome the impairments before regular communications can be established. Determining the appropriate values for optical pre-compensation parameters can be time consuming.
In addition, when the path of the communications link between the transmitter and the receiver is not the assumed path or is changed over time, for example via optical switches, the optical impairments of the path such as the dispersion profile will not match a particular design profile. As a result of changes in the path, the margin or reach of the optical transmission may be reduced or may preclude the operation of a communications link altogether. It is also often desirable to establish a new optical connection swiftly, in an interval of tens or hundreds of milliseconds. Until the optical parameters such as dispersion are approximately known, at-speed digital communication cannot be established. The process of re-initializing the optical communications link is a time consuming process.
When the optical parameters such as chromatic dispersion of a link is unknown, an approximate value must be determined in order to compensate the impairment before the link can be successfully started. One common solution is to directly measure the impairment of the optical link using specialized test equipment during commissioning of the optical communications link. Another solution is to scan slowly, and have operators at nodes on both ends of the communications link until signal is acquired. Both solutions are time consuming and require communications between the ends of the link. Neither of these solutions are capable of dealing with sudden changes in the optical link which may introduce new impairments.
Accordingly, a method of asynchronously and independently initializing an optical communication links between nodes of a optical communications system utilizing transmitters adapted to pre-compensate link impairments based on an optical compensation parameter remains highly desirable.